U.S. patent number 5,147,592 [Application Number 07/580,218] was granted by the patent office on 1992-09-15 for process of and apparatus for preparing optical recording medium substrates.
This patent grant is currently assigned to Canon Kabushiki Kaisha. Invention is credited to Hisanori Hayashi, Hirofumi Kamitakahara, Osamu Kanome, Tsuyoshi Santoh, Tetsuya Sato, Hitoshi Yoshino.
United States Patent |
5,147,592 |
Sato , et al. |
September 15, 1992 |
Process of and apparatus for preparing optical recording medium
substrates
Abstract
A process for preparing a substrate sheet for optical recording
mediums includes the steps of extruding a molten resin to form a
resin sheet and bringing the resin sheet into pressure between a
preformat roll and a roll disposed opposingly to said preformat
roll to transfer a preformat pattern to the surface of the resin
sheet, wherein the roll disposed opposingly to the preformat roll
includes a roll whose surface is covered with an elastomeric
resin.
Inventors: |
Sato; Tetsuya (Kawasaki,
JP), Kanome; Osamu (Kawasaki, JP), Yoshino;
Hitoshi (Tokyo, JP), Kamitakahara; Hirofumi
(Yokohama, JP), Hayashi; Hisanori (Kawasaki,
JP), Santoh; Tsuyoshi (Yokohama, JP) |
Assignee: |
Canon Kabushiki Kaisha (Tokyo,
JP)
|
Family
ID: |
16975724 |
Appl.
No.: |
07/580,218 |
Filed: |
September 10, 1990 |
Foreign Application Priority Data
|
|
|
|
|
Sep 12, 1989 [JP] |
|
|
1-234747 |
|
Current U.S.
Class: |
264/167; 264/106;
264/210.2; 264/211.12; 425/327; 425/363; 425/377; 425/810;
264/1.34 |
Current CPC
Class: |
B29C
43/222 (20130101); B29D 17/007 (20130101); B29C
48/08 (20190201); B29C 48/001 (20190201); B29L
2017/00 (20130101); B29C 48/0018 (20190201); B29C
48/07 (20190201); Y10S 425/81 (20130101) |
Current International
Class: |
B29C
43/22 (20060101); B29C 47/00 (20060101); B29C
69/02 (20060101); B29D 17/00 (20060101); B29C
047/00 (); B29C 047/88 () |
Field of
Search: |
;264/1.3,106,107,167,175,177.1,177.17,210.2,211.12,216,284,296
;425/224,327,363,377,810 ;100/155R,168,169
;29/121.1,121.6,129.5,132 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
Primary Examiner: Tentoni; Leo B.
Attorney, Agent or Firm: Fitzpatrick, Cella, Harper &
Scinto
Claims
We claim:
1. A process for preparing a substrate sheet for optical recording
mediums, comprising the steps of extruding a molten resin from a
T-die to form a resin sheet and pressing said resin sheet in a
substantially molten state between a preformat roll and a roll
disposed opposingly to said preformat roll to transfer a preformat
pattern to a surface of the resin sheet such that said resin sheet
is brought into direct contact with the circumferential surface of
said roll disposed opposingly to said preformat roll while said
resin sheet is maintained at a temperature higher than a surface
temperature of said roll disposed opposingly to the preformat roll,
wherein
said roll disposed opposingly to the preformat roll comprises a
metal roll a surface of which is covered with an elastomeric resin
having a thickness ranging from 0.1 mm to 5 mm.
2. A process for preparing a substrate sheet for optical recording
mediums according to claim 1, wherein said elastomeric resin
comprises a silicone resin.
3. An apparatus for preparing a substrate sheet for optical
recording mediums, comprising a means for extruding a molten resin
from a T-die to form a resin sheet; a preformat roll and a roll
disposed opposingly to the preformat roll between which said resin
sheet is pressed to transfer a preformat pattern to a surface of
the resin sheet; a means for bringing said resin sheet into direct
contact with the circumferential surface of said roll disposed
opposingly to said preformat roll; and a means for heating said
resin sheet to a temperature higher than a surface temperature of
said roll disposed opposingly to the preformat roll, wherein;
said roll disposed opposingly to the preformat roll comprises a
metal roll substrate, a surface of which is covered with an
elastomeric resin having a thickness ranging from 0.1 mm to 5
mm.
4. An apparatus for preparing a substrate sheet for optical
recording mediums according to claim 3, wherein said elastomeric
resin comprises a silicone resin or a fluorine resin.
5. An apparatus for preparing a substrate sheet for optical
recording mediums, comprising a means for extruding a molten resin
to form a resin sheet; a preformat roll and a roll disposed
opposingly to the preformat roll between which said resin sheet is
pressed to transfer a preformat pattern to the surface of the resin
sheet; a means for bringing said resin sheet into direct contact
with the circumferential surface of said roll disposed opposingly
to said preformat roll; and a means for heating said resin sheet to
a temperature higher than a surface temperature of said roll
disposed opposingly to the preformat roll, wherein;
said roll disposed opposingly to the preformat roll is provided on
its surface with a cover comprised of a material having a low
thermal conductivity and an elasticity; said cover being effective
as a heat insulating layer against the resin sheet, which
suppresses conduction of heat of said resin sheet to the rolls so
that said resin sheet has a double refraction value lower than a
double refraction value produced by heat dissipation from the resin
sheet to the rolls when said resin sheet is pressed between the
preformat roll and a roll whose surface is not provided with said
cover.
6. A process for preparing a substrate sheet for optical recording
mediums according to claim 1, wherein said elastomeric resin
comprises a fluorine resin.
7. An apparatus for preparing a substrate sheet for optical
recording mediums according to claim 3, wherein said elastomeric
resin comprises a fluorine resin.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a process for preparing a
substrate used for a high-density optical recording medium such as
an optical disk or an optical card. It also relates to an apparatus
for preparing the substrate.
2. Related Background Art
Substrates for optical recording mediums such as CDs, laser disks,
DRAW disks and photomagnetic disks that optically record and
reproduce information have been hitherto prepared by injection
molding or compression molding. Although the substrates are
required to have a high flatness or smoothness, these molding
methods tend to cause warpage or inclusion of bubbles, and hence
the detection of information by light may be greatly inhibited.
Thus, great difficulties are involved in controlling temperature
and pressure, assuring accuracy of a mold, or preventing generation
of bubbles. An apparatus with a large scale is also required,
resulting in a great increase in cost. Moreover, substrates are
formed sheet-by-sheet in the compression molding, and hence
complicated post-processing is required, resulting in a poor
productivity.
Use of a plastic flat sheet enables easy manufacture of a smooth
and uniform sheet with less inclusion of bubbles, and hence a
method has been proposed in which a stamper is brought into close
contact with this plastic flat sheet under application of pressure
to transfer grooves to the sheet. However, a very high pressure
must be used in order to apply pressure over the whole disc surface
of the flat sheet.
As a means for solving this problem, a plastic sheet extruded from
an extruder may be passed between a molding roll provided with a
stamper in close adhesion and a mirror-finished roll so that
grooves formed on the stamper can be thereby formed on the flat
sheet under a low pressure, and then center holes and peripheries
may be trimmed away. Substrates for optical recording mediums can
be thus obtained using an apparatus of a small scale.
Incidentally, as materials for the substrates used for high-density
optical recording mediums such as optical disks, polycarbonate
resins have been mainly utilized in view of their properties such
as impact properties and weathering properties.
Polycarbonates, however, have the disadvantage that they tend to
cause double refraction and the double refraction may immediately
occur when a strain is present. The occurrence of double refraction
causes impairment of a C/N ratio at the time of
recording-reproducing, and therefore must be decreased as far as
possible.
As a means for solving this problem, the present applicants have
proposed, in Japanese Patent Application No. 63-289828 filed Nov.
16, 1988, a process for preparing a substrate for an optical
recording medium, comprising transferring a preformat pattern to a
resin sheet extruded from a die, by the use of a three-roll system,
wherein the temperature and rotational speed of each of three rolls
are controlled so that the double refraction in the resin sheet can
be decreased.
Employment of this process has made it possible to obtain a
substrate for optical recording mediums that causes less double
refraction. However, more than just to decrease the double
refraction, it is sought to uniformly decrease double refraction
over the whole surface of a substrate.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a process for
preparing a substrate sheet for optical recording mediums, that can
prepare in a good mass productivity a substrate sheet for optical
recording mediums on which fine preformat patterns of micron order
can be transferred with accuracy and in which double refraction has
been uniformly decreased.
Another object of the present invention is to provide an apparatus
for preparing a substrate sheet for optical recording mediums, that
can prepare in a good mass productivity a substrate sheet for
optical recording mediums on which fine preformat patterns of
micron order can be transferred with accuracy and in which double
refraction has been uniformly decreased.
Taking account of the above objects, the present inventors made
various studies. They noted the fact that, when a resin sheet is
pressed between rolls, the heat of the resin sheet is transferred
to a metallic roll having a higher thermal conductivity and
consequently the resin sheet is cooled to a temperature lower than
its glass transition temperature, as well as the fact that when a
resin sheet is pressed between rolls, a strain is produced because
of the pressure applied to the resin sheet. As a result of the
studies, they found that these facts are responsible for the double
refraction of a substrate sheet for optical recording mediums, and
thus have accomplished the present invention.
The process of the present invention for preparing a substrate
sheet for optical recording mediums is characterized by a process
for preparing a substrate sheet for optical recording mediums,
comprising the steps of extruding a molten resin to form a resin
sheet and bringing said resin sheet into pressure between a
preformat roll and a roll disposed opposingly to said preformat
roll to transfer a preformat pattern to the surface of the resin
sheet, wherein;
said roll disposed opposingly to the preformat roll comprises a
roll whose surface is covered with an elastomeric resin.
The apparatus of the present invention for preparing a substrate
sheet for optical recording mediums is characterized by an
apparatus for preparing a substrate sheet for optical recording
mediums, comprising a means for extruding a molten resin to form a
resin sheet, and a preformat roll and a roll disposed opposingly to
the preformat roll between which said resin sheet is pressed to
transfer a preformat pattern to the surface of the resin sheet,
wherein;
the surface of said roll disposed opposingly to the preformat roll
is covered with an elastomeric resin.
According to the present invention, the elastomeric resin cover
formed on the surface of a roll disposed opposingly to a preformat
roll acts as a heat insulation layer and can maintain the
temperature of the resin sheet to a temperature higher than the
glass transition temperature when the resin sheet formed by
extrusion is pressed between the rolls. Thus, it is presumed that a
substrate sheet for optical recording mediums with less double
refraction can be obtained since the resin sheet is fixed in the
state in which no strain is applied thereto.
It is also presumed that a substrate sheet for optical recording
mediums that causes less strain can be obtained since the covering
of a roll surface with an elastomeric resin prevents the sheet from
being forcibly rolled when pressed between the rolls.
BRIEF DESCRIPTION OF THE DRAWINGS
FIGS. 1 and 2 are schematic illustrations of the embodiments of the
apparatus for preparing substrate sheet for optical recording
mediums according to the present invention.
FIG. 3 is an enlarged view of a roll having on its surface an
elastomeric resin cover.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
The present invention will be described below with reference to the
drawings.
FIGS. 1 and 2 each schematically illustrate an embodiment of the
process for preparing a substrate sheet for optical recording
mediums according to the present invention.
According to the preparation process shown in FIG. 1, a resin
serving as a material for a substrate is melt-extruded using a melt
extruder 1 to form a resin sheet 2 which serves as a substrate
sheet for optical recording mediums and which is extruded through
T-die 12. The resulting resin sheet 2 is pressed between a
preformat roll 3 having on its surface a preformat pattern and a
roll 4 disposed opposingly to the preformat roll and covered on its
surface with an elastomeric resin, and the preformat pattern is
transferred to the surface of the resin sheet 2. A substrate sheet
for optical recording mediums can be thus prepared.
The preparation process shown in FIG. 2 is a process in which a
preformat pattern is transferred to the resin sheet 2 by the use of
three rolls which are horizontally arranged. In FIG. 2, the numeral
1 denotes an extruder; 2, a resin sheet; 3, a second roll which is
a preformat roll having on its surface a preformat pattern; 4, a
first roll whose surface is covered with an elastomeric resin; 5, a
mirror-finished pressure roll; and 6, a take-up unit.
The distances between the rolls 3 and 4 and between the rolls 3 and
5 are made controllable so that the preformat pattern on the second
roll 3 can be well transferred to the surface of the resin sheet 2.
The resin sheet 2 extruded from the extruder 1 through T-die 12 is
inserted between the rolls 3 and 4 and between the rolls 3 and 5 in
a softened state, where the preformat pattern on the second roll 3
and the mirror surface of the pressure roll 5 are successively
transferred. A substrate sheet for optical recording mediums can be
thus formed.
FIG. 3 is an enlarged view of the roll 4 disposed opposingly to the
preformat roll 3 and having on its surface an elastomeric resin
cover. In FIG. 3, the numeral 7 denotes a roll substrate; and 8, an
elastomeric resin with which the roll substrate 7 is covered. The
elastomeric resin used in the present invention should preferably
have a thermal resistance since the roll 4 itself is heated and the
resin sheet almost in a molten state comes into contact with the
roll 4. For example, when polycarbonate is used as the resin, the
roll 4 is heated to have its surface temperature of from
120.degree. C. to 150.degree. C. and is brought into contact with a
polycarbonate resin sheet of about 250.degree. C. Hence, the
elastomeric resin should preferably have a thermal resistance
against a temperature of 200.degree. C. or more, and particularly
250.degree. C. or more.
The surface of the roll covered with the elastomeric resin is
required to be mirror-finished to a certain degree in order to
improve the accuracy of the substrate to which preformat patterns
are transferred. For this purpose, the surface of the elastomeric
resin should preferably be polished. More specifically, its surface
accuracy should preferably be controlled to be 3 .mu.m or less, and
particularly 1 .mu.m or less.
The elastomeric resin that can meet the above requirements may
preferably include silicone resins and fluorine resins.
This elastomeric resin cover may be desirably formed so as to
function as a heat insulation layer. Namely, if a metallic roll
having no elastomeric resin cover 8 is used as the roll 4, the
resin sheet 2 comes into contact with a metallic roll 4 having a
good thermal conductivity when pressed between rolls. As a result,
the heat of the resin sheet 2 is mostly transferred to the roll 4
and cooled, and as the resin sheet 2 is fixed in that state it
retains the strain produced when it is pressed between rolls, so
that a substrate sheet for optical recording mediums with a large
double refraction is formed. However, the heat insulation layer
comprised of the elastomeric resin cover can control the cooling of
the resin sheet. More specifically, it can prevent heat dissipation
from the resin sheet 2 when the resin sheet is pressed between
rolls, so that the strain produced in the resin sheet 2 pressed
between rolls does not remain and a substrate sheet for optical
recording mediums with a low double refraction can be obtained.
The covering of the roll surface with the elastomeric resin can
bring about a certain degree of relaxation of the pressure applied
to the resin sheet, so that a strain produced in the resin sheet
can be decreased. In addition, a resin sheet extruded from a T-die
has an irregular thickness, and hence the pressure applied to a
thick portion becomes much larger, resulting also in a large strain
produced in the resin sheet. Against this problem also, the
covering of the roll with the elastomeric resin can achieve a
certain degree of absorption of the thickness irregularity, so that
the pressure applied to the resin sheet can be made uniform.
The elastomeric resin cover 8 can suppress irregularity of double
refraction when its thickness is so designed as to enable
relaxation of the pressure applied to the resin sheet 2 and also to
achieve uniform conduction of heat from the roll to the resin cover
surface. Stated specifically, the thickness may preferably be set
in the range of from 0.1 to 5 mm, and particularly from 0.5 to 3
mm, in approximation.
In the present invention, the double refraction (single path) of
the substrate sheet may preferably be 20 nm or less, and
particularly 15 nm or less, over the whole surface in a case of
using the light having a wavelength used in recording-reproducing.
This is because the fluctuation of reflectance becomes smaller and
the S/N ratio at the time of reproducing can be improved for an
optical recording medium on which the light is made incident
through a substrate to carry out recording and/or reproducing.
In the roll 4, a metal is used for the material used for the roll
substrate 7. For example, stainless steel, chromium steel,
chromium-plated steel or the like can be preferably used. This roll
substrate 7 may also preferably be mirror-finished on its
surface.
A process for preparing a substrate sheet for optical recording
mediums according to the present invention will be described below
in detail with reference to FIG. 2.
In the present invention, resin pellets fed into an extruder 1 are
first heated and melted in a barrel of the extruder 1, pressed
forward by a screw provided inside the extruder, and then formed
into a sheet through a T-die 12. In order to enable accurate
transfer of the preformat pattern, the T-die may preferably be so
disposed that a resin sheet is extruded to the gap between the
preformat roll 3 and the roll 4.
Next, the resin sheet extruded from the T-die is pressed between
the preformat roll 3 and the roll 4, and thus the preformat pattern
is transferred. Here, the resin sheet 2 may preferably be almost in
a molten state. This is due to the fact that the resin can thereby
be sufficiently pressed into the uneven surfaces of the stampers
and hence fine patterns can be accurately transferred. For this
reason, the T-die may preferably be kept heated to a temperature as
high as possible so long as the resin may not burn or scorch, and
may preferably be heated to a glass transition temperature of the
resin (hereinafter "Tg") +110.degree. C. to Tg+200.degree. C., and
particularly Tg+130.degree. C. to Tg+190.degree. C. In the case of
polycarbonate resins, for example, it may preferably be heated to
from 260.degree. C. to 340.degree. C., particularly from
280.degree. C. to 330.degree. C., and more particularly from
290.degree. C. to 320.degree. C. In addition, since the preformat
pattern may not be well transferred it tends to cause double
refraction when the resin sheet is cooled between the T-die and
preformat roll, the distance between the T-die and the point at
which the resin sheet is pressed between the preformat roll 3 and
the first roll 4 may preferably be 20 cm or less, particularly 15
cm or less, and more particularly 10 cm or less. Their surrounding
atmosphere should also preferably be kept at a temperature of
60.degree. C. or higher.
The apparatus may also preferably have the constitution of vertical
extrusion as shown in FIG. 2, where the resin sheet is pressed
between the rolls at the point vertically beneath the T-die so that
the resin sheet can be correctly extruded to that point. This is
for the reason that the vertical extrusion enables more correct
extrusion, than horizontal extrusion, of the resin sheet to the
point it is pressed between the rolls since the resin is nearly in
a molten state.
The surface temperatures of the preformat roll 3 and the rolls 4
and 5 may vary depending on the resin to be used. When, for
example, a polycarbonate resin is used, the temperature of the
preformat roll may be set in the range of from 135.degree. to
145.degree. C., and the temperature of the third roll 5, usually,
from 140.degree. to 150.degree. C., taking account of the heat
distortion temperature of from 140.degree. to 150.degree. C. of the
polycarbonate. The temperature of the first roll having on its
surface the elastomeric resin cover may be set to a temperature of,
for example, from 90.degree. to 120.degree. C., which is lower than
that of the preformat roll so that the resin sheet may be readily
moved forward in the state it has come into close contact with the
preformat roll 3.
In the process for preparing the optical recording medium of the
present invention, the preformat roll having on its surface a
preformat pattern can be prepared by fastening on a mirror-finished
roll substrate a nickel stamper used in the manufacture of a
conventional substrate for an optical recording medium, or by
forming by photolithography a preformat pattern directly on a roll
substrate or on a pattern-forming layer provided on a roll
substrate.
An ordinary mirror-finished roll can be used as the third roll 5.
As roll substrates for the rolls 3 and 5, a chromium-plated steel
product or the like can be used.
The roll diameters and peripheral speeds of the rolls 3, 4 and 5
may vary depending on the thickness of the substrate sheet for
optical recording mediums to be formed. Taking account of the
transfer accuracy of a preformat pattern to the resin sheet at the
time the resin sheet is pressed between rolls, the temperature
control of the resin sheet or the prevention of accumulation of
heat to the resin cover of the roll 4, the diameter of a roll
should preferably be from 200 to 450 mm, and particularly from 250
to 350 mm when the resin sheet has a thickness of from 0.3 to 2 mm.
The peripheral speed of a roll should preferably be from 0.5 to 5
m/min., and particularly from 1 to 4 m/min.
The preformat pattern the preformat roll 3 of the present invention
has on its surface specifically refers to a pattern corresponding
with, for example, tracking grooves used for optical disks or
optical cards, which groove(s) may be spiral, concentrically
circular, or parallel, and have a width of from 0.5 .mu.m to 2
.mu.m, a pitch of 1.0 .mu.m to 5 .mu.m and a depth of from 200
.ANG. to 5,000 .ANG., in approximation. It may also refer to a
pattern corresponding with tracking grooves used for optical disks
or optical cards, which groove(s) may be spiral, concentrically
circular, or parallel, and have a width of from 2 .mu.m to 5 .mu.m,
a pitch of 8 .mu.m to 15 .mu.m and a depth of from 200 .ANG. to
5,000 .ANG., in approximation.
As resins used as materials for the substrate, thermoplastic resins
having a high transmission to the light used for recording and
reproduction are preferred, including, for example, acrylic resins,
polyester resins, polycarbonate resins, vinyl resins, polysulfone
resins, polyolefin resins, and cellulose derivatives.
As described above, the present invention makes it possible to;
(1) obtain substrates for optical recording mediums that have less
optical anisotropy and are uniform;
(2) obtain substrate sheets for optical recording mediums on which
preformat patterns have been transferred with good accuracy;
(3) continuously carry out the formation of a resin sheet and
transfer of preformat patterns, and obtain substrates for optical
recording mediums in a good mass productivity and low cost; and
(4) readily control the cooling of a resin sheet because of the
heat insulation layer on the roll surface, and thus readily prepare
substrates for optical recording mediums with a low double
refraction.
EXAMPLES
The present invention will be specifically described below by
giving Examples.
EXAMPLE 1
Using the apparatus as shown in FIG. 2, a substrate sheet for
optical recording mediums was prepared.
As the first roll 4, a roll substrate of 300 mm in diameter, made
of steel material was covered with a silicone resin (SE1188,
trademark for a product of Toray Silicone Co., Ltd.) in a thickness
of 2 mm, and its surface was mirror-finished.
As the preformat roll 3, a stamper of 0.2 mm thick made of nickel
and having a preformat pattern was fastened with screws on a
mirror-finished roll substrate of 300 mm in diameter, made of
chromium-plated steel. The preformat pattern was a pattern
corresponding with a tracking groove of a spiral form, having a
groove width of 0.6 .mu.m, a pitch of 1.6 .mu.m and a depth of
1,000 .ANG., used for an optical disk of 13 cm in diameter.
As the third roll 5, a mirror-finished roll of 300 mm in diameter,
made of chromium-plated steel was used.
A polycarbonate resin (trade name: L-1250; a product of Teijin
Chemicals Co., Ltd.) was extruded from the extruder 1 to form the
resin sheet 2 of 300 mm in width, which was then pressed between
the first roll 4 and the preformat roll 3 so that the preformat
pattern was transferred to the surface of the polycarbonate resin
sheet 2. A substrate sheet of 1.2 mm thick for optical disks was
thus prepared. The temperature of the T-die was controlled to be
300.degree. C.
The surface temperature of the first roll 4 was controlled to be
180.degree. C.; that of the preformat roll 3, to be 140.degree. C.;
and that of the third roll 5, to be 135.degree. C. The rotational
speed of each roll was controlled to be 2 m/min.
On the substrate sheet for optical disks thus prepared, double
refraction values at 15 points arbitrarily selected within the
region in which a preformat pattern was transferred were measured
to reveal that the values were 15 nm or less at all points. The
preformat pattern was confirmed to have been transferred in a good
state.
COMPARATIVE EXAMPLE 1
Example 1 was repeated to prepare a substrate sheet for optical
disks, except that the first roll 4 used in Example 1 was replaced
with a mirror-finished roll of 300 mm in diameter, made of
chromium-plated steel.
The resulting substrate sheet for optical disks was evaluated in
the same manner as in Example 1 to reveal that the values of double
refraction ranged from 5 to 50 nm, which were very non-uniform.
EXAMPLE 2
As the first roll 4, a roll substrate of 300 mm in diameter, made
of steel was mirror-finished. Thereafter, the resulting roll
substrate was covered with a heat-shrinkable tubing (a product of
Gunze Ltd.) of 1 mm thick made of a fluorine resin PFA and then
heated at 150.degree. C. to cause the tube to shrink. Thereafter,
the surface of the tube was polished to have a surface accuracy of
2 .mu.m or less.
As the preformat roll 3, a roll substrate of 300 mm in diameter,
made of chromium-plated steel was mirror-finished. Thereafter, a
stamper of 0.2 mm thick made of nickel and having a preformat
pattern on its surface was fastened by a mechanical means on the
surface of the mirror-finished roll substrate. The preformat
pattern was a pattern corresponding with tracking grooves having a
groove width of 3 .mu.m, a pitch of 12 .mu.m and a depth of 3,000
.ANG., used for an optical card on which the grooves were formed in
parallel in a longitudinal direction in a rectangular region of 54
mm in length and 85 mm in breadth.
As the third roll 5, a mirror-finished roll of 300 mm in diameter,
made of chromium-plated steel was used. The apparatus as shown in
FIG. 2 for preparing a substrate sheet for optical cards was thus
made up.
Next, a polycarbonate resin (trade name: L-1250; a product of
Teijin Chemicals Co., Ltd.) was extruded from the extruder 1 to
form the resin sheet 2 of 250 mm in width, which was then pressed
between the first roll 4 and the preformat roll 3 so that the
preformat pattern was transferred to the surface of the
polycarbonate resin sheet 2. A substrate sheet of 0.4 mm thick for
optical cards was thus prepared. The temperature of the T-die was
controlled to be 310.degree. C.
The surface temperature of the first roll 4 was controlled to be
120.degree. C.; that of the preformat roll 3, to be 145.degree. C.;
and that of the third roll 5, to be 130.degree. C. The rotational
speed of each roll was controlled to be 4 m/min.
On the substrate sheet for optical cards thus prepared, double
refraction values at 15 points arbitrarily selected within the
region in which a preformat pattern was transferred were measured
to reveal that the values were 20 nm or less at all points. The
preformat pattern was confirmed to have been transferred in a good
state.
COMPARATIVE EXAMPLE 2
Example 2 was repeated to prepare a substrate sheet for optical
cards, except that the first roll 4 used in Example 2 was replaced
with a mirror-finished roll of 300 mm in diameter, made of
chromium-plated steel.
The resulting substrate sheet for optical cards was evaluated in
the same manner as in Example 2 to reveal that the values of double
refraction ranged from 5 to 50 nm, which were very non-uniform.
EXAMPLE 3
As the first roll 4, a roll substrate of 250 mm in diameter, made
of steel was mirror-finished. Thereafter, the resulting roll
substrate was covered with a heat-shrinkable tubing (a product of
Gunze Ltd.) of 0.3 mm thick made of a fluorine resin PFA and then
heated at 120.degree. C. to cause the tube to shrink. Thereafter,
the surface of the tube was polished to have a surface accuracy of
1 .mu.m or less.
As the preformat roll 3, a roll substrate of 250 mm in diameter,
made of chromium-plated steel was mirror-finished. Thereafter, a
stamper of 0.1 mm thick made of nickel and having a preformat
pattern on its surface was fastened with screws on the surface of
the mirror-finished roll substrate. The preformat pattern was a
pattern corresponding with tracking grooves having a groove width
of 3 .mu.m, a pitch of 12 .mu.m and a depth of 3,000 .ANG., used
for an optical card on which the grooves were formed in parallel in
a longitudinal direction in a rectangular region of 54 mm in length
and 85 mm in breadth.
As the third roll 5, a mirror-finished roll of 250 mm in diameter,
made of chromium-plated steel was used. The apparatus as shown in
FIG. 2 for preparing a substrate sheet for optical cards was thus
made up.
Next, a polycarbonate resin (trade name: L-1225; a product of
Teijin Chemicals Co., Ltd.) was extruded from the extruder 1 to
form the resin sheet 2 of 300 mm in width, which was then pressed
between the first roll 4 and the preformat roll 3 so that the
preformat pattern was transferred to the surface of the
polycarbonate resin sheet 2. A substrate sheet of 0.4 mm thick for
optical cards was thus prepared. The temperature of the T-die was
controlled to be 300.degree. C.
The surface temperature of the first roll 4 was controlled to be
120.degree. C.; that of the preformat roll 3, to be 150.degree. C.;
and that of the third roll 5, to be 135.degree. C. The rotational
speed of each roll was controlled to be 3 m/min.
On the substrate sheet for optical cards thus prepared, double
refraction values at 15 points arbitrarily selected within the
region in which a preformat pattern was transferred were measured
to reveal that the values were 15 nm or less at all points. The
preformat pattern was confirmed to have been transferred in a good
state.
COMPARATIVE EXAMPLE 3
Example 3 was repeated to prepare a substrate sheet for optical
cards, except that the first roll 4 used in Example 3 was replaced
with a mirror-finished roll of 250 mm in diameter, made of
chromium-plated steel.
The resulting substrate sheet for optical cards was evaluated in
the same manner as in Example 3 to reveal that the values of double
refraction ranged from 5 to 50 nm, which were very non-uniform.
In the measurement of double refraction in the above Examples 1 to
3 and Comparative Examples 1 to 3, a semiconductor laser was
adjusted to emit a beam of 1 mm in diameter, and the beam was made
incident on the molded substrate sheets to measure the degree of
polarization of the light transmitted through them. Light with a
wavelength of 830 nm was used.
* * * * *